What is the reason for swelling in both legs although serum ?

Millions of causes: Your complain is very general. Things that cause that include obesity, allergic reactions, heart failure, severe malnutrition, severe fluid overload, vein disease, etc. Swelling is a very non-specific finding and has so many causes that identifying the cause takes more than 400 words allows me to help. If it is causing you problems, then getting a doctors opinion should give you peace of mind.